1. Technical Field
The present invention relates to an electro-optical element and a scanning optical apparatus.
2. Related Art
In projection image display apparatuses in recent years, an electric-discharge lamp such as a super-high pressure mercury lamp is generally used as a light source. However, such an electric-discharge lamp has problems in that, for example, a durable life is relatively short, instantaneous lighting is difficult, a color reproducibility range is narrow, an ultraviolet ray irradiated from the lamp may deteriorate a liquid crystal light bulb. Thus, there is proposed a projection image display apparatus in which a laser beam source that irradiates monochromatic light is used instead of such an electric-discharge lamp (see, for example, JP-A2003-75767).
As the projection image display apparatus in which such a laser beam source is used, there is an image display apparatus of a laser scan type in which scanning means (a scanner) is used. The scanning means used in the image display apparatus of the laser scan type is required to realize both high-speed scanning and a large deflection angle. In particular, it is necessary to scan a laser beam at a scanning speed of several tens kHz to display video signals having formats such as VGA (Video Graphics Array), XGA (Extended Graphics Array), and HDTV (High Definition Television). Thus, in general, an image display apparatus in which a resonant MEMS (Micro Electro Mechanical System) scanner is used as scanning means is adopted because a deflection angle of 15° to 30° can be expected.
The MEMS scanner still has problems described below. As a first problem, since the MEMS scanner is a resonant scanner, the MEMS scanner can perform only a sinusoidal reciprocating action at a non-uniform rate. As a second problem, since the MEMS scanner has a high Q value, it is impossible to obtain a practical deflection angle when a frequency deviates from a resonance frequency. Therefore, it is necessary to accurately control a resonance frequency of a system including the MEMS scanner or change a driving frequency following a change in the resonance frequency of the system. The accurate control of a resonance frequency is extremely difficult technically. The change of a driving frequency needs to be precisely synchronized with a second axis of the MEMS scanner.
As a third problem, since the MEMS scanner has a limit in a scanning speed, display at a resolution such as 4 k (4096×2160) of the DCT (Digital Cinema Initiatives) specifications is difficult. As an example of scanning means other than the MEMS scan, there are an acoustic optical scanner and an electro-optical scanner. These scanners have a small scanning deflection angle compared with that of the MEMS scanner and cannot obtain a practical scanning deflection angle.
Thus, in order to obtain a large deflection angle using the electro-optical scanner, as shown in FIG. 8A, it is conceivable to set a dimension N between electrodes of an element 101 long. However, when the dimension N between the electrodes is set long, it is necessary to apply a larger voltage to the electrodes in order to generate an electric field same as that generated when the dimension N between the electrodes of the element 101 is snort. As a result, power consumption is increased to cause an increase in cost. Thus, as shown in FIG. 8B, it is conceivable to set a dimension between electrodes short in order to perform low-voltage drive while setting a dimension M of an element 102 in a traveling direction of light long in order to obtain a large deflection angle. However, with such a structure, light that has traveled the inside of the element 102 and has been bent downward by a refractive index distribution collides with a lower electrode 105 when the dimension M of the element 102 is too long. Therefore, the light that has traveled the inside of the element 102 is not emitted from an emitting end face of the element 102.